Use of chromeless phase shift features to pattern large area line/space geometries

ABSTRACT

Method for using chromeless phase shift lithography (CPL) masks to pattern large line/space geometries and corresponding CPL masks. The method comprises using a short wavelength light to illuminate a CPL mask comprising a reticle having a plurality of phase-shifting features interspersed with non-phase-arranged in a substantially alternating two-dimensional pattern. When light passes through the phase-shifting features it is phase-shifted relative to light passing through the non-phase-shifting areas of the CPL mask. The phase-shifted and non-phase-shifted light passing through the reticle is then projected onto a resist layer applied over a semiconductor substrate. The resultant composite aerial image intensity distribution is such that an area of the resist having a shape defined by a periphery of a corresponding pattern of phase-shifting features is sufficiently exposed to pattern a large area feature in the resist. Subsequent semiconductor processing operations may then be performed to pattern a corresponding feature in the semiconductor substrate.

FIELD OF THE INVENTION

[0001] The field of invention relates generally to semiconductors and,more specifically but not exclusively relates to a method for patterninglarge area line/space geometries in semiconductor substrates through theuse of chromeless phase shift masks.

BACKGROUND INFORMATION

[0002] Chromeless phase shift lithography (CPL) has been investigatedfor many years as a possible single-mask resolution enhancementtechnique for lines/spaces in semiconductor devices. For positiveresists, it is particularly well suited to the patterning ofsemi-isolated narrow lines but not to dense line/spaces or contacts.However, with significant mask design effort and added mask complexity,contacts and semi-dense line/spaces have been successfully patterned.Like other phase shifting techniques such as alternating PSM, CPL canprovide significantly better aerial image contrast compared to binarymasks; unlike alternating PSM, however, it is a single mask singleexposure technique avoiding many of the dual-reticle concerns such asthroughput, mask layout, and reticle to reticle overlay.

[0003] CPL uses phase edges between 0 and 180° phase shift regions onthe mask to pattern lines along the phase edges. This is possiblewithout chrome because destructive interference of light diffracted fromregions immediately on either side of the phase edge result in an aerialimage minimum at the wafer corresponding to the phase edge withexcellent contrast if it is isolated enough. With just one phase edgedefining lines, it would be impossible to pattern arbitrary layoutswithout a second mask to clear unwanted phase edges. CPL allows one toavoid using a second mask by patterning narrow lines with two closelyspaced parallel phase edges that cannot be resolved. The combined aerialimage of the two parallel phase edges is still a deep single minimumwhich patterns as one line but now the “line” on the reticle (mask) canbe drawn just as it would with chrome, wherein the chrome is replaced bya phase shifted region. However, this only works for lines that are notwide; if the phase shifted line becomes too wide, i.e. the two phaseedges of the line move too far apart, then they become individuallyresolvable and will pattern as two parallel lines. If the phase shiftedline is too narrow, the aerial image contrast gets worse very quickly asthe phase shifted region become smaller and looks more like a uniformpiece of quartz. These two cliffs constrain the size of phase shiftlines to a relatively tight range of small widths.

[0004] These effects are illustrated in the aerial image diagram of FIG.1, and the schematic diagrams of FIGS. 2A and 2B. FIG. 1 corresponds toan aerial image intensity distribution simulation of a CPL reticle thatis illuminated with 193 nm light using on-axis quadrupole illumination(0.1 sigma poles at 0.7 sigma radii), and projected using a 0.68 NA(numeric aperture) projection lens. The ideal case corresponds to a 0.1μm separation, which produces a deep single minimum. As the separationwidth increases, the aerial image results in a pair of minimums beingproduced, as shown by the 0.2 μm and 0.5 μm separation curves. Forexample, a separation of 0.5 μm would result in two lines beingresolved. This of course is undesired. As a result, wider lines aretypically patterned using a binary (i.e., chrome-patterned) reticle.

[0005] The results of the foregoing phenomenon are shown schematicallyin FIGS. 2A and 2B. FIG. 2A depicts an aerial image 200 produced by aCPL mask 202 that includes a narrow phase-shifting feature comprising amesa 204 having a width W₁. After the resist is exposed, developed, andwashed, and a metal layer is deposited over a substrate 206, and singleline 208 is formed. The single line 208 has a width W₁′ thatsubstantially matches the width W₁ of narrow mesa 204 (or is otherwiseproportional thereto for projection systems that employ magnification).As illustrated in FIG. 2B, when short wavelength light is directed at aCPL mask 210 including a wide mesa 212 phase-shifting feature with awidth W₂, the resulting aerial image 214 includes two narrow peaksrather than one wide peak. As a result, two lines 216 and 218 are formedinstead a desired wide signal line.

[0006] Under conventional practices, this wide line width/featurelimitation of CPL is addressed by providing a mask that employs both CPLfeatures and chrome features. The CPL phase shift features are used toproduce narrow features, while chrome patterns are added to the CPL maskto produce large area features such as wide lines and pads on thesemiconductor substrate. In this instance, the chrome is used to blocklight rather than phase shift the light, as is well-known in the art.One disadvantage of this approach is that the mask making processbecomes more complex. Extra lithographic and etch steps in the maskmaking process are required to make both the chrome features and the CPLfeatures. In addition the chrome and CPL patterns need to be preciselyaligned.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The foregoing aspects and many of the attendant advantages ofthis invention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified:

[0008]FIG. 1 is a diagram illustrating an aerial image of two phaseedges vs. phase edge separation for a CPL mask.

[0009]FIG. 2A is a schematic diagram illustrating an aerial image andresulting single line formed from a CPL feature having a small width;

[0010]FIG. 2B is a schematic diagram illustrating an aerial image andresulting pair of lines formed from a CPL feature having a larger width;

[0011]FIGS. 3A and 3B illustrate CPL masks for patterning large arealine/space geometries, wherein the CPL mask of FIG. 3A employs aplurality of phase-shifting features comprising mesas formed over aquartz substrate, and the CPL mask of FIG. 3B employs a plurality ofphase-shifting features comprising recesses formed in a quartzsubstrate;

[0012]FIG. 3C is a cross-section view of the CPL mask of FIG. 3A takenalong section cut 13C-13C;

[0013]FIG. 4A is a plan view of a CPL mask pattern comprising aplurality of mesas formed on a quartz reticle and arranged in acheckerboard pattern to pattern a large pad in accordance with anembodiment of the invention;

[0014]FIG. 4B illustrates a cross-section view of the CPL mask of FIG.4A, taken along section cut 4B-4B;

[0015]FIG. 5A is a plan view of a CPL mask pattern comprising aplurality of recesses formed in a quartz reticle and arranged in acheckerboard pattern to pattern a large pad in accordance with anembodiment of the invention;

[0016]FIG. 5B illustrates a cross-section view of the CPL mask of FIG.5A, taken along section cut 5B-5B;

[0017]FIGS. 6A, 6B, 6C, and 6D illustrate various optional phase-shiftedpattern configurations suitable for patterning large area line/spacegeometries;

[0018]FIGS. 7A and 7B illustrate various feature shapes that may bepatterned using the phase-shift pattern configurations illustrated inFIGS. 6B and 3A, respectively; and

[0019]FIG. 8 is a schematic diagram illustrating exemplaryphotolithography process in which a CPL mask corresponding to theteachings of the invention may be employed to pattern large arealine/space geometries;

[0020]FIG. 9A, 9B, and 9C respectively show various stages post-exposureoperations performed during a semiconductor manufacturing process,wherein FIG. 9A illustrates a semiconductor substrate configurationafter exposed resist as been removed, FIG. 9B illustrates the processstage after a layer of metal has been deposited over areas of thesemiconductor substrate from which resist has been removed; and FIG. 9Cillustrates the process stage after the remaining unexposed resist hasbeen removed; and

[0021]FIG. 10 is a diagram illustrating resist profiles produce by aconventional chrome mask feature technique and CPL techniques inaccordance with embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0022] Embodiments of method and apparatus for patterning large arealine/space geometries areas using chromeless phase shift techniques aredescribed herein. In the following description, numerous specificdetails are set forth to provide a thorough understanding of embodimentsof the invention. One skilled in the relevant art will recognize,however, that the invention can be practiced without one or more of thespecific details, or with other methods, components, materials, etc. Inother instances, well-known structures, materials, or operations are notshown or described in detail to avoid obscuring aspects of theinvention.

[0023] Reference throughout this specification to “one embodiment” or“an embodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

[0024] Integrated circuits (IC) are manufactured from a semiconductorsubstrate, such as a silicon wafer, using a series of processing steps.Generally, the various electronic elements (e.g., transistors, gates,etc.) of the IC are first formed using processing steps particular tothe type of transistor being employed by the chip. For example, for CMOS(complementary metal oxide) IC's, these steps include depositing variouslayers, combined with various lithography steps, etching steps, andimplantation steps to form the electronic elements. These electronicelements are then “integrated” via conductive (e.g., copper, aluminum,etc.) lines parallel to the substrate surface and contacts perpendicularto the surface. In addition to these metal layer features, IC's alsoinclude features such as pads and the like.

[0025] Oftentimes, the width of various features will vary. For example,a modern IC may have many lines having a base width corresponding to thelimitation of the photolithography technology (e.g., 0.25 microns),while other lines and features such as pads have a width that is severalmultiples of the base line width. As discussed above, under conventionaltechniques, the narrow base lines may be patterned using phase-shiftingfeatures on a CPL mask, while the larger line width and area featuresare patterned using corresponding chrome features on the reticle. Thisuse of the two different mask technologies is required due to thelimitation of conventional CPL techniques for patterning large linewidth and feature areas, as exemplified in FIGS. 1 and 2B above.

[0026] In accordance with aspects of the invention, the conventional CPLlarge pattern area/line width limitation may be overcome by patterning aCPL mask with a plurality of phase shifting features interspersed withnon-phase-shifting areas of the mask and arranged in a substantiallyalternating two-dimensional pattern. In general, the phase-shiftingfeatures may comprise recesses or mesas, which are formed in a quartzsubstrate via a suitable manufacturing process (e.g., via etching). Forexample, CPL masks 300 and 300′ in FIGS. 3A and 3B illustrate CPL maskssuitable for patterning a target area on a substrate having a shapesubstantially corresponding to the overall perimeter area occupied bythe phase-shifting feature (mesa or recess) pattern on the mask. In oneembodiment, CPL mask 300 includes a plurality of mesas 302 extendingupward from a quartz substrate 304. Optionally, a plurality of recesses302′ may be used in place of the mesas, as depicted by CPL mask 300′ inFIG. 3B. Generally, the height of the mesas (or depth of the recesses)is selected such that light impinging on the mesa (or recess) areas willhave a phase shift of 180° relative to light passing through the mask inareas unoccupied by the mesas (or recesses), which comprisenon-phase-shifting areas of the mask.

[0027] If the phase-shifting features are small enough and close enough,the corresponding composite aerial image produced by projecting thephase-shifted and non-phase-shifted light will merge to providesufficient exposure to pattern a large area in a resist layer. Forexample, the result of the phase shift affect caused by thephase-shifting features of CPL mask 300 produces a composite aerialimage 306 (after projection) shown in FIG. 3C. This aerial imageintensity distribution may then be used to pattern a wide line area 308on a target area of a semiconductor substrate 310.

[0028] Plan views of phase-shifting feature patterns for patterninglarger exemplary geometries are shown in FIGS. 4A and 5B. FIG. 4Adepicts a CPL mask 400 comprising a plurality of mesas 402 extendingupward for a quartz reticle 404. As shown in FIG. 4B, as light ray 406passes through a phase-shifting mesa, it is shifted in phaseapproximately 180° relative to a light ray 408 that passes through anon-phase-shifting area of reticle 400. A similar phase-shift affect maybe produced by a recessed feature in a reticle. For example, as shown bythe cross-section view of the CPL mask 500 of FIG. 5A, a light ray 508passing through a recess 502 formed in a reticle 504 is shifted in phase180 ° relative to a light ray 506 passing through a non-phase-shiftingarea of the reticle.

[0029] Optionally base patterning configurations are shown in FIGS.6A-6D. The option A pattern of FIG. 6A comprises a plurality of squares600 configured in a rectangular array. In option B, the areacorresponding to all of the outside squares in the checkerboard areacomprises a phase-shifting feature. Option C is similar to option B,except that the center square 602 along each edge now corresponds to anon-phase-shifting area. Finally, option D comprises a sort of “bull'seye” configuration, wherein a single square 604 is formed within a phaseshifted closed perimeter 606 separated by a non-phase-shifted area 608.

[0030] Although illustrated in the foregoing examples as squares,various other shapes may be employed, such as rectangles, diamonds, etc.Generally, the shape of the phase-shifting features can take any shapeas long as they are physically small enough and placed close enoughtogether to ensure that the aerial images of the features merge toprovide a combined aerial image capable of pattern the area as a largeresist structure. Additionally, the size of the shapes employed shouldbe selected such that the critical dimension (which will generally bethe longest dimension) of the shape ensures that the desired patternsubstrate pattern is obtained. For example, the length of a rectangularelement should be less than a feature length that causes multiple linesto be patterned. Generally, the selected shape should be is configuredso as to produce a two dimensional pattern having alternatingphase-shifting features interspersed with and non-phase-shifting areasof the reticle. In FIGS. 3A, 3B, and 6A-D, these non-phase-shifted areasare depicted by the white space between the cross-hatched areas, whichrepresent the phase-shifting features.

[0031] In general, a base patterning configuration can be extended topattern shapes of various target configurations. For example, phaseshifting patterns for patterning an “L”-shaped feature and a “T”-shapedfeature are respectively illustrated by CPL mask 700 and 702 of FIGS. 7Aand 7B. The perimeter (i.e., outline) of the target large resist areawill generally map to a corresponding perimeter drawn around the generalarea occupied by phase-shifting features on the mask (corresponding to aparticular target feature), as shown by the dashed-line perimeters 704and 706 drawn around phase-shifting feature patterns 708 and 710 in FIG.7A and 7B, respectively. As a result, through use of appropriateillumination and projection components (such as described below),phase-shifting feature pattern 708 of CPL mask 700 can be used to exposean “L”-shaped resist area 712 on a resist layer 714 shown in FIG. 7A.Similarly, phase-shifting feature pattern 710 can be used to expose a“T”-shaped resist area 716 on a resist layer 718 shown in FIG. 7B.

Exemplary Implementation of a CPL Mask for Patterning Large AreaLine/Space Geometries

[0032] A lithography process corresponding to an exemplaryimplementation of a CPL mask in accordance with aspect of the inventionis shown in FIG. 8. The process is performed using a lithographyapparatus (i.e., stepper) the employs a short wavelength illuminationsource 800 to emits light at a wavelength appropriate for the process(e.g., 248 nm, 193, or 157 nm). In general, such an illumination sourcewill be used to illuminate a reticle having phase-shifting featuresformed thereon. These illumination sources may include on-axisillumination sources and off-axis illumination sources such as annular,dipole, quadrupole, and quasar light sources. Generally, off-axisillumination in combination with CPL enables the patterning of finerfeatures for a given set of optical parameters (e.g., λ, k₁, and NA).For example, in FIG. 8 the light produced by illumination source 800 isredirected by optical element 802 to produce a quadrupole light source.In this case, the quadrupole poles are positioned off the optical axisof the lithography apparatus in such a way that most of the light fromthese sources impinges on the reticle of CPL mask 702 at an acute anglewith little light impinging perpendicular to the reticle.

[0033] As discussed above, light impinging on the phase-shiftingfeatures (in this case mesas corresponding to phase-shifting featurepattern 710) on the reticle is shifted 180° in phase when it passesthrough the reticle. Meanwhile, light impinging on non-phase-shiftingareas of the reticle (i.e., the “white” space around and interspersedbetween the cross-hatched phase-shifting features of CPL mask 702)passes through the reticle without a phase shift. Both the phase-shiftedand non-phase-shifted light then is passed through a projection system808, depicted as a lens 809 disposed behind an aperture 810 forsimplicity; in practice, the projection system may typically employother optical components that are not shown. The projection system isused to focus the light toward a focal area in which a semiconductorsubstrate 812 coated with layer 814 (e.g., via spin coating) of resistis placed, as further shown in the blown-up detail of FIG. 8. Thisproduces an aerial image intensity distribution that comprises thecomposite of the aerial image intensity distributions for the individualphase-shifting features.

[0034] Some portions 816 of the resist 814 (indicated by the denselycrosshatched areas) are exposed to higher intensity light rayscorresponding to the composite aerial image, while other portions 818(indicated by the lightly cross-hatched areas) are not, based on thephase-shifting feature pattern formed on CPL mask 702, in combinationwith various optical considerations, such as numeric aperture (NA), thewavelength A of the light, the amount of offset, the arrangement of theoptical components, etc.

[0035] After the resist has been exposed, one or more processing stepsare performed to develop away the exposed resist while leaving theunexposed areas of the resist. In accordance with positive tone resistcharacteristics, exposure of the resist with sufficient light intensitycauses a chemical change that makes it soluble in developer fluid. Theresist so exposed is developed away by dissolving in an appropriatesolvent. The results of this process are shown in FIG. 9A. Next, a metaldeposition operation is performed to deposit a layer of metal 900 on thesurface of semiconductor substrate 812 in areas from which the resisthas been removed. The end result of this operation is depicted in FIG.9B. Finally, the unexposed portion 818 of the resist is removed using anappropriate chemical process, leaving the substrate appearing as shownin FIG. 9C. Thus, the foregoing process enables a large area geometry tobe patterned in a resist layer by means of a CPL mask, and,subsequently, a corresponding metal layer feature having a configurationdefined by the resist layer pattern is then deposited onto thesemiconductor substrate to form the large area feature.

Simulated Results

[0036]FIG. 10 contains a graphic comparing the patterning results of aconventional chrome mask, the CPL mask configured in the checkerboardphase array pattern of FIG. 3A, and a CPL mask configured in a patterncorresponding to option B illustrated in FIG. 6B. As can be readilyobserved, both of these CPL mask options produce a resist pattern thatis similar to that produced using the conventional chrome patterningtechnique. As a result, CPL mask in accordance with the teachingsdisclosed herein may be made to pattern both fine line width and finepitch features in combination with larger line width features and areas,such as pads, without requiring the use of chrome in the mask.

[0037] The above description of illustrated embodiments of theinvention, including what is described in the Abstract, is not intendedto be exhaustive or to limit the invention to the precise formsdisclosed. While specific embodiments of, and examples for, theinvention are described herein for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize.

[0038] These modifications can be made to the invention in light of theabove detailed description. The terms used in the following claimsshould not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims. Rather, thescope of the invention is to be determined entirely by the followingclaims, which are to be construed in accordance with establisheddoctrines of claim interpretation.

What is claimed is:
 1. A method for patterning a large area geometry,comprising: illuminating a chromeless phase shift lithography (CPL) maskcomprising a reticle having a plurality of phase-shifting featuresinterspersed with non-phase-shifting areas with a short wavelength lightsource, wherein light passing through the phase-shifting features isphase-shifted relative to light passing through the non-phase-shiftingareas of the CPL mask and wherein the phase-shifting features arearranged in a substantially alternating two-dimensional pattern withnon-phase-shifting areas of the CPL mask; and projecting phase-shiftedand non-phase-shifted light passing through the CPL mask onto a layer ofresist.
 2. The method of claim 1, wherein the plurality ofphase-shifting features are arranged in a base pattern comprising acheckerboard pattern.
 3. The method of claim 1, wherein the plurality ofphase-shifting features are arranged in a base pattern comprising acheckerboard pattern having a substantially filled outside boarder. 4.The method of claim 1, wherein the plurality of phase-shifting featuresare arranged in a base pattern comprising an island, surrounded by anon-phase shifting area, which in turn is surrounded by a phase-shiftingperipheral area.
 5. The method of claim 1, wherein the plurality ofphase-shifting feature are arranged in a rectangular array.
 6. Themethod of claim 1, wherein CPL mask is illuminated with an off-axisillumination source.
 7. The method of claim 1, wherein the off-axisillumination source comprises a quadruple illumination source.
 8. Themethod of claim 1, wherein the phase-shifting features comprise aplurality of recesses formed in the reticle.
 9. The method of claim 1,wherein the phase-shifting features comprise a plurality of mesasextending above non-phase-shifting areas of the reticle.
 10. The methodof claim 1, wherein the large area geometry has a shape defined by apolygon having at least five sides.
 11. The method of claim 1, whereinthe short wavelength light source produces light having a wavelength ofone of 248, 193, or 157 nanometers.
 12. A method for patterning a largearea feature on a semiconductor substrate, comprising: providing achromeless phase shift lithography (CPL) mask comprising a reticlehaving a plurality of phase-shifting features interspersed withnon-phase-shifting areas, said phase-shifting features arranged in asubstantially alternating two-dimensional pattern with non-phaseshifting areas, said pattern substantially occupying an area on the CPLmask having a shape corresponding to the large area feature;illuminating the CPL mask with a short wavelength light source, whereinlight passing through the phase-shifting features is phase-shiftedrelative to light passing through the non-phase-shifted areas of the CPLmask; and projecting phase-shifted and non-phase-shifted light passingthrough the CPL mask onto a layer of resist applied over thesemiconductor substrate to expose an area on the resist corresponding tothe large area feature.
 13. The method of claim 12, wherein CPL mask isilluminated with an off-axis illumination source.
 14. The method ofclaim 12, wherein the off-axis illumination source comprises a quadrupleillumination source.
 15. The method of claim 12, wherein thephase-shifting features comprise a plurality of recesses formed in thereticle.
 16. The method of claim 12, wherein the phase-shifting featurescomprise a plurality of mesas extending above non-phase-shifting areasof the reticle.
 17. The method of claim 12, further comprising:developing the resist layer to remove areas of the resist exposed to asufficient exposure intensity corresponding to the projected aerialimage intensity distribution; depositing a metal layer in areas on thesemiconductor from which the resist is removed; and removing remainingportions of the resist layer.
 18. A chromeless phase shift lithography(CPL) mask comprising: a reticle having a plurality of phase-shiftingfeatures formed therein, said plurality of phase-shifting featurescausing light passing therethough to be shifted approximately 180° inphase relative to light passing through non-phase-shifting areas of thereticle not occupied by a phase-shifting feature, said phase-shiftingfeatures arranged in a substantially alternating two-dimensional patternwith the non-phase-shifting areas of the CPL mask to produce a projectedaerial image to pattern one or more large resist areas on asemiconductor substrate.
 19. The method of claim 1, wherein theplurality of phase-shifting features are arranged in a base patterncomprising a checkerboard pattern.
 20. The method of claim 1, whereinthe plurality of phase-shifting features are arranged in a base patterncomprising a checkerboard pattern having a substantially filled outsideboarder.
 21. The method of claim 1, wherein the plurality ofphase-shifting features are arranged in a base pattern comprising anisland, surrounded by a non-phase shifting area, which in turn issurrounded by a phase-shifting peripheral area.
 22. The CPL mask ofclaim 18, wherein the plurality of phase-shifting feature are arrangedin a rectangular array.
 23. The CPL mask of claim 18, wherein at leastone of said one or more large resist areas is non-rectangular.
 24. TheCPL mask of claim 18, wherein the plurality of phase-shifting featurescomprise a plurality of recesses formed in the quartz substrate.
 25. TheCPL mask of claim 18, wherein the plurality of phase-shifting featurescomprise a plurality of mesas extending upward from the quartzsubstrate.
 25. The CPL mask of claim 18, wherein reticle comprisesquartz.
 26. The CPL mask of claim 18, wherein the phase-shiftingfeatures are substantially square in shape.
 27. The CPL mask of claim18, wherein the plurality of phase-shifting features are arranged in apattern occupying a peripheral area having a shape defined by a polygonhaving at least five sides.